I noticed, that "suddenly" (read: it was not that way before) the "2","3" and "R/S" got kinda "sticky" feeling when pressed (damn is this difficult
to explain...lets try)
The key itsself (what is touched with my thumb) is NOT sticky. But when the key is pressed down it sounds/feels that the key needs to be
torn away from something with some "higher" force. This only happens, when the key has rest for a while and after this "initial" press of the key the "stickyness" goes away until the key has again rested for some time.
It may be, that something gets into the mechanic of the keys somehow (dont ask me about the "how"-part of "somehow.... )

I had opened the calculator, carefully removed the pcb and cleaned the surface of the backside of the keys and the folio (covering the metal
domes) with a cotton stick (word? Q-Tips I mean). But it does not help that much.
It seems, that the tiny tip on the back of the keys, which directly presses onto the dome isn't able to move easily over the surface of the folio anymore.
Any safe ideas how to clean the keys/the affected surface of the folio more intensily without risk any damage to this wonderful gem of math?

Is there any glue to hold parts of the keyboard in place or a printing
which get accidentally solved since IPA is a good solvent ?

I can't think of any parts of the calculator that IPA would dissolve. As a test, I just liberally sprayed the front and back of the keyboard and the sticky-backed plastic film covering the key domes with IPA from a spray bottle with no ill effects. Admittedly, I tested it on a piece of 3M Scotch Magic Tape first. It doesn't even dissolve the glue on the sticky side on the tape, which might be bad news for you, depending on the cause of your sticky key issue!